The convective afterdrop component during hypothermic exercise decreases with delayed exercise onset.
OBJECTIVE
Following cold water immersion, the post-cooling decrease in esophageal temperature (Tes) (i.e., afterdrop) is 3 times greater during exercise than during shivering, presumably due to increased muscular blood flow and convective core-to-periphery heat loss with exercise (J. Appl. Physiol. 63:2375, 1987). We felt that if exercise were to commence once the afterdrop period during shivering is complete, the threat of a further decrease in Tes (i.e., a second afterdrop) during the subsequent exercise would be minimized because much of the convective capacity for core cooling would already be dissipated.


METHODS
Six subjects were each cooled three times in 8 degrees C water, until Tes decreased to 35.3 +/- 0.7 degrees C, and rewarmed by either shivering alone, exercise, or exercise commencing once a shivering afterdrop period was complete.


RESULTS
The initial afterdrop was greater during Exercise only (1.1 +/- 0.4 degrees C) than Shivering only (0.35 +/- 0.3 degrees C) and Shivering-Exercise (0.45 +/- 0.2 degrees C) (p < 0.05). In contrast, exercise caused a secondary afterdrop of only 0.38 +/- 0.3 degrees C during Shivering-Exercise (p < 0.05). The initial rewarming rate during Exercise only (3.45 degrees C.h-1) was greater than the initial (2.7 degrees C.h-1) and second (2.4 degrees C.h-1) rewarming rates during Shivering-Exercise (p < 0.05), but not significantly greater than during Shivering only (2.99 degrees C.h-1) (p < 0.1).


CONCLUSIONS
It is likely that during the Shivering-Exercise protocol, continued blood flow to shivering muscles: a) contributes to the initial afterdrop, and thus b) diminishes the convective capacity (or heat sink) available for further cooling during subsequent exercise.
Giesbrecht
G G
GG
Laboratory for Exercise and Environmental Medicine, Health, Leisure and Human Performance Research Institute, Faulty of Physical Education and Recreation Studies, Manitoba, Canada.
Bristow
G K
GK
D000328:Adult; D001823:Body Composition; D001827:Body Height; D001831:Body Temperature / Q000502:physiology*; D001835:Body Weight; D018481:Convection; D004947:Esophagus / Q000503:physiopathology*; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D005260:Female; D006801:Humans; D007035:Hypothermia / Q000209:etiology / Q000503:physiopathology*; D007101:Immersion / Q000009:adverse effects*; D008297:Male; D017694:Rewarming / Q000379:methods*; D012768:Shivering / Q000502:physiology*; D012885:Skinfold Thickness; D013997:Time Factors